Compound semiconductors of the Groups III-V and II-VI, as well as IV-IV, of the Periodic Chart of the elements, are normally synthesized at high temperatures and even very high pressures, as a boule. Typically at the melting point and standard pressure, the partial pressure of the Group V(or VI) element is high, such that special precautions are required to keep the crystalline imperfections low. For example, in the case of Gallium Arsenide (GaAs) at the melting point of 1238 C., the partial pressure of As over the liquid GaAs is approximately one atmosphere (Reference 1). This is also important in the case of epitaxial layer growth of compound semiconductors, where typical temperatures of 600-1050 C. are used to form many technologically important alloys and devices (Reference 2). Gallium Nitride, has a projected melting point of over 2500 C. with the Nitrogen overpressure of 10 thousand atmospheres (Reference 3).
Compound semiconductors have achieved commercial success during the past twenty years in high brightness Light Emitting Diodes (L.E.D's), for lighting, high performance lasers for optical fiber applications, high efficiency solar cells for satellite power, high speed transistors (in particular Hetero Bipolar Transistors) for cell telephones and other electronic and optoelectronic devices.
Blue LED's and lasers are of particular importance to not only to complete the optical spectrum but for very high density D.V.D and other optical storage applications. A particularly difficult problem for these materials relates to the substrate necessary to grow thin layers that comprise the laser, L.E.D or other electronic or optoelectronic device (Reference 4). The substrate performs several functions from providing the mechanical support, to thermal management, to allowing epitaxy to take place through its crystal structure and dimensions, to being either electrically active through impurity doping or insulating again possibly through impurity doping. Group III-Nitride substrates are the ideal materials for homoepitaxy of these materials. It is known that the growth of large (over a few mm in diameter) single crystal substrates, is extraordinarily difficult to achieve compared to GaAs or InP, for example, which are commercially available to 150 mm Outside Diameter (OD).
This application, in part, relates to one currently available growth process which produces free-standing GaN substrates and is called Hydride Vapor Phase Epitaxy or HVPE (Reference 5). In this process, a sacrificial substrate such as Sapphire, is used to deposit GaN or AlN or their alloys. Inside the apparatus, Group V source elements are carried into a heated zone by using the Group V Hydrides, while for the Group III, a mixture of Hydrogen Chloride in Hydrogen is passed over the Group III metal (e.g Gallium or Aluminum). This process can produce a 100 micron thick substrate in about an hour and easily as large as 75 mm in OD (Reference 5). Typically this type of grown GaN layer contains a very large number of crystalline defects (dislocations) due to the lattice and thermal mismatch. These are seen by a microscope and also revealed through acid etching, as pits, hence, Etch Pit Density (EPD). In this case, the EPD is in the 108 to 1010 per cm2 or even higher.
Ion implantation into a compound semiconductor crystal material is well known that levels in the 10+16/cm2 range and higher will result in an amorphous phase (Reference 6). This amorphous phase will recrystallize into a polycrystalline material at annealing temperatures below 1100 C. It is necessary to anneal out the implantation damage at temperatures exceeding two thirds of the melting point of GaN which is 2518 C. (Reference 7).
Light energy transfer techniques such as from a flash lamp do not produce a fast enough rise in the substrate temperature and are limited to the top temperatures they can achieve to about 1200 C., which is not sufficient to anneal out ion implantation damage of compound semiconductor materials. Directed energy beams such as Pulsed electron beams were used in the past to anneal ion implantation damage in Silicon wafers as large as 100 mm OD. The pulsed electron beam, typically of 0.1 microseconds in duration, produced by an electron gun or a capacitor discharge, is accelerated through a 100 KV field and directed at an optimized angle on the substrate. Alternatively, a pulsed laser, such as a Neodymium pumped YAG laser, is also used. The electron beam total is in the range of 800 to 1000 Amperes and the electrons acquire approximately 10 KeV energy. The pulsed electron beam as above melts the Silicon wafer surface at 1410 C. and the crystallinity of the top micron or so is repaired.
In the case of compound semiconductor materials, a directed energy beam on a non protected surface will result in worse crystallinity due to decomposition.